I know the lead times on television drama make these things difficult but it would have been nice, given the prevailing gloom, to have a cheerful Robin Hood back on our screens.

Tricky, perhaps, given that the love of his life died at the end of the last series, but he could have reacted to that blow with brave resolution and offered us a hero who cares nothing for material things. This is a man, after all, who doesn't catch a bag of coins when it's flung at him but pierces it with a single shot, so that it showers groats on the grateful churls beneath its flight path. Surely he should have stormed back on to the screen as an exemplary figure for our times, blithely whisking the barons' contractual bonuses from under their noses and sharing it out among the smaller shareholders. Instead, for the first 40 minutes anyway, he's a clinically depressed Dark Knight Robin, bent on vengeance and bitterly cynical about his wealth-redistribution mission statement.

The vengeance thing is a bit unconvincing, frankly. He swears blind that he wants to kill Guy of Gisborne, but seems suspiciously reluctant to take advantage of the numerous opportunities that present themselves. At one point, Guy seizes a passing child as a human shield and hares off up a nearby hill. Robin is about 10 yards behind him, but despite the considerable weight handicap Guy has to cope with Robin still hasn't closed the gap 400 vertical feet further on. He also flubs a perfect opportunity to finish the job later, pinning Guy to a wooden door by his coat sleeves, but steering well clear of all vital organs. At which point one has to ask oneself – is Robin in reality a lousy shot (and the moneybag thing was just a fluke) or is he deliberately trying to miss? If I was one of the Merry Men I would be beginning to get a bit paranoid about Robin's reluctance to close the deal when it came to high-value targets, even though he's ostensibly back to business as far as the rich/poor thing goes.

There is competition for the tea-time tosh market now, because Primeval has returned, a series which, like Robin Hood, keeps one toe dipped in the pool of self-mockery just in case anyone gets too pompous about it. For Robin Hood, the safety rope is Keith Allen announcing that an imminent execution is "a great day for homeland security". In Primeval, it's Ben Miller, drily undermining the notion that it all might actually be in earnest. For me, though, they don't self-mock nearly enough, particularly given a disregard for lowest-common denominator plausibility, which even a seven-year-old might have problems with. In this episode, for example, the authorities were fretting at one point about possible civilian witnesses, despite the fact that a giant crocodilian dinosaur had just stampeded through central London during the rush hour, consuming a traffic warden as a mid-morning snack. Not only would it have been noticed by hundreds of people within 30 seconds of leaving the British Museum, but the mobile- phone footage would have been posted on YouTube five minutes after that. In these things, surely, the big fibs don't matter (that Eocene dinosaurs can burst through glitterball "anomalies" into our universe), but the little fibs (that nobody but the experts would notice them) nibble away at the pleasure of the thing.

Primeval was all cod Egyptology, but Timewatch – Pyramid: the Last Secret offered the real thing with a film about Jean-Pierre Houdin, who gave up his architectural practice, sold his house and moved into a tiny apartment with a fold-up bed in order to devote virtually all his waking hours to working out how the Egyptians had built the Great Pyramid at Giza. This sounded like a full-scale nervous breakdown to me, but Bob Brier, an American Egyptologist, thought Houdin might be on to something with his theory about how the "yooge stone blocks" had been winched into place. Houdin thinks there's a hidden construction ramp inside (the Great Pyramid). "If he's right," said Brier, "it's the greatest discovery since Tutankhamun," which rather puts Sir Alexander Fleming in his place. Houdin had a lot of theories and stupendous amounts of fancy computer graphics to back up his ideas, but not a lot in the way of hard, incontrovertible evidence. Apparently, the Egyptians aren't keen to let him dismantle the Great Pyramid to prove it one way or the other. And intriguingly the film turned out to have a kind of hidden chamber itself – the conspicuous name-checks for the Dassault computer software that had allowed Houdin to map out his ideas finally being explained by the revelation, in the final credits, that it was produced by PSL Productions and Dassault Systèmes. "We Egyptologists are sceptical by nature," said an American scholar who intended to sit on the fence until Houdin came up with something more than vivid speculation. We television critics are, too, and this programme aroused the instinct to the full.